Selector and image rejector circuit



Sept. 16, 1941. F CUTTWG A I 2,256,277

SELECTOR AND IMAGE REJECTOR CIRCUIT Filed Aug. 3, 1940 ATTORNEY Patented Sept. 16, 1941 S'ELECTGR. AND IBIAGE REJECTOR CIRCUIT Fulton Cutting and Verlis Wiley, Buffalo, N. 'Y., assignors to Colonial Radio Corporation,

Buifalo, N. Y.

Application August 3, 1340, Serial No. 350,667

(Cl. 25i)20) Claims.

This invention relates to selector and image rejector circuit particularly adapted for use in superheterodyne radio receivers.

Radio receivers of the superheterodyne type are known to be subject to a form of interference known as image interference. caused by the reception of a signal differing from the desired signal by twice the value of the intermediate frequency of the receiver and many schemes have been suggested for eliminating or reducing it. These schemes usually have involved the provision of at least two tuned circuits, one of which is tuned to the desired signal and the other is tuned to the image signal. We have discovered that it is possible to reduce image interference without the use of circuits tuned to the image frequency, thereby reducing the cost and simplifying the apparatus for a given standard of performance.

.Among the objects of our invention may be mentioned:

To provide a radio receiver in which the selecting circuits are so arranged as to produce a high ratio of signal to image without requiring the provision of special image trap circuits tuned to image frequency or the like.

To provide such a circuit which may be tuned either by a variable air condenser as usual or by a variable inductance of the moving iron core type and which is adapted for use in home receivers and automobile receivers as well.

To provide such a circuit which may be utilized as a coupling circuit between the tubes of a radio frequency amplifier as well as in the input circuit of the first tube.

Still other objects and advantages of our invention will be apparent from the specification.

In this application, we have particularly pointed out and distinctly claimed the part, improvement or combination which we claim as our invention or discovery and we have explained the principles thereof and the best mode in which we have contemplated those principles, so as to istinguish our invention from other inventions.

In the drawing:

Fig. 1 is a circuit diagram of our invention applied in the input circuit of the first tube of a radio receiver;

Fig. 2 is a circuit diagram of our invention showing our circuit applied as a coupling circuit between a pair of tubes; and

Figs. 3 and 4 are Vector diagrams of the action of the circuit, Fig. 3 being the vector diagram at image frequency and Fig. 4 a similar diagram at desired signal frequency; and

This is electrode 9 to the other terminal thereof.

Fig. 5 is a diagram as shown in Fig. l, but showing another form of gauging of the controls.

Referring now more particularly to Fig. l, i designates the antenna or signal collector of any suitable type, and 5 the first tube of the receiver, the tube being of any suitable type but herein shown as a pentode comprising cathode 1, heated by heater filament 8 and comprising control grid 9 and anode ill, the cathode 1 being grounded.

Other types of tubes, such as triodes, pentagrid converters and the like may be employed, but since these are well-known in the art and the operation of our invention is not affected by the use of different types of tubes, the simple type herein shown is thought to be suflicient for illustration.

The circuit may comprise inductance 2 and condensers 3 and 4 connected to form a closed loop circuit. The antenna may be connected to one terminal of the inductance 2 and the control The common point of condensers 3 and 4 may be connected to ground through condenser 5. All of these elements may be made variable ifdesired. For example, condenser 3 which ordinarily is the antenna trimming condenser will preferably be made adjustable and set once and for all and then left alone. Condenser 4 may be the usual variable air tuning condenser and inductance 2 may be fixed or may be variable as by an iron core which may be ganged with the other tuning elements to be moved in and out of coil 2 to vary its inductance. Condenser 5 may likewise be made variable.

In operation the input circuit is tuned to the desired signal frequency and as will be noted the voltage impressed between the control electrode 9 and cathode 1 is the sum of the voltages appearing across condensers 4 and 5. Referring now to the vector diagram, Fig. 3 for image frequencies, let EA represent the image frequency voltage generated in the antenna. This image frequency may be considerably higher than the frequency to which the input circuit is tuned. For example, if the intermediate frequency of the receiver is 455 kilocycles, then the image frequency will be 910 kilocycles higher than the signal frequency. This image frequency voltage causes a current to flow in the antenna circuit, which current may be designated as IA and may be assumed to flow in series through condensers 3 and 5 to ground. Since this circuit is purely capacitive, the current will be ahead of the voltage which causes it, as indicated in the dia gram.

This antenna current produces a voltage across condenser 3 and also across condenser and as will be understood, these voltages which may be designated E3, and E5 will be in phase with each other and will be 90 behind the current which produces them; that is to say, they will be in phase with the antenna voltage.

The voltage E3, tends to produce a current flowing in the closed loop circuit formed by condenser 3 inductance 2 and condenser 4, but this circuit is tuned to a frequency much lower than the image frequency and consequently, its reactance to image frequency is inductive and the loop circuit current due to image frequency voltage will be 90 behind the voltage which produces it, as shown by the vector I1.,. This loop circuit current flowing through condenser 4 produces a voltag across the same and this voltage lags 90 behind the current which produces it and may be represented by the vector E4 It will now beobserved that the volt-ages across condensers 4 and 5 for image frequency are 180 out of phase and it will also be observed that the magnitude of these two voltages may be controlled without aifecting their phase by adjustment of condensers 4 and 5. The effect of this is that by properly adjusting condensers 4 and 5 with respect to each other, the magnitude of by means of condenser 4 or by means of inductance 2, it will be found that theoretically zero image frequency voltage can only be obtained at one frequency, and as the tuning is varied the Voltage vectors at image frequency,

while still 180 out of phase, are not of the proper magnitude to cancel each other and thus there will be some voltage impressed upon the 1 grid. However, this will ordinarily not be serious and the adjustment is made for best image reduction at the frequency for which the worst image interference would otherwise occur, if it is not desired to vary 4 or 5 in operation.

However, the circuits can'be made to track exactly that is, to preserve substantially zero image frequency for all values of signal by ganging condenser 5 with the tuning element which is made variable for tuning purposes. For instance, if 4 is the tuning condenser, condenser 5 may be made variable and ganged with condenser 4 as in Fig. 5. Similarly, if inductance 2 is the tuning element, condenser 5 may be ma de variable and ganged with inductance 2 as in Fig. 1. The proper value of condenser5 for any desired signal frequency may be determined by adjusting the tuning and the value of condenser 5 for a number of signal frequencies spread over the desired frequency band, from which adjustments the value which condenser 5 should have these voltages may be made substantially equal v and being 180 out of phase their vector sum will be substantially .zero; that is to say, by

properly adjusting condensers 4 and 5 it is possible to reduce the image voltage appearing on the grid of tube 6 to substantially zero at any one image frequency. If the frequency changes, either condenser 4 or condenser 5 must be readjusted to obtain an exact balance.

Considering now Fig. 4, the vector diagram for signal frequencies, it will be observed that there is as before an antenna voltage EA of signal frequency, this producing as before an antenna current IA leading the voltage by 90 and this in turn producing a voltage E3 across condenser 3 and a second voltage E5 across condenser 5, these voltages as before being in phase with each other and lagging 90 behind the antenna current and thus being in phase with the antenna voltage; As before, the voltage E's causes a current to flow in the closed loop circuit consistingof condenser 3, inductance 2 and condenser 4, but since this circuit is resonant to signal frequency,

its current at signal frequency will now b in phase with the voltagegcausing it, and in phase with the voltage E3 and E5 and may be represented by ILS. This current flowing through condenser 4 producesa voltage across it, which voltage lags the current by 90 andmay be represented by Eg.

The voltage applied to the grid is the vector sum of E and E4 and may be represented by the vector sum of these two voltages as En By comparison of Figs. 3 and 4, it will be observed that the desired signal produces a voltage between the cathode and grid oftube 6 whereas substantially zero voltage is produced for image frequency. In passing, it may be pointed out that Figs. 3 and 4 cannot be superposed or drawn on the same axes, because they represent different frequencies, and are to be considered as rotating at differentangular velocities.

r If condenser 5 is adjusted initially to produce zero image frequency at one value of signal, and thereafter left alone while the circuit is tuned for any signal frequency may be determined and condenser 5 arranged accordingly,

Referring now to Fig. 2 in which we have shown the circuit arranged as a coupling circuit between tubes, there is provided the first tube 6 having its plate circuit connected through an impedance! to a source of B voltage, and connected through acoupling condenser [3 to the circuit comprising inductance 2, condenser 3,-condenser 4 and condenser 5, having one side grounded. As before, the second tube l5 may have its grid I6 connected between condenser 4 and inductance 2 and its cathode l1 connected to ground. The operation of the circuit in this arrangement is essentially the'sarne as already described with reference to Fig. l,- and it may be noted that when the circuit is used in interstage position as well as in the input circuit, the variable elements for tuning and image suppression may be ganged together in both circuits; that is, each circuit may be ganged as before and the two circuits may be ganged together.

While we have shown and described certain preferred embodiments of our invention, it will be understood that modifications and changes may be made Without departing from the spirit and scope of our invention, as will be clear to those skilled in the art.

We claim: l

1. An image rejector and signal selector circuit formed of a single tuned circuit only comprising the combination with a vacuum tube amplifier having a cathode and control grid, of an inductance, a pair of condensers connected in series with each other and with said inductance, and

them are substantially equal and opposite for at least one value of image frequency.

2. An image rejector and signal selector circuit formed of a single tuned circuit only comprising, in combination, an inductance, a pair of condensers connected in series with each other across said inductance to form a closed loop circuit, a third condenser having one terminal connected to the common point of said series connected condensers, an antenna circuit connected across said third condenser and one of said series connected condensers and feeding energy to said tuned circuit through one path only, a vacuum tube amplifier having a cathode and a control grid, the control grid-cathode circuit of said amplifier being connected across said third condenser and the other of said series connected condensers, said circuit being tunable to desired signal frequency and the values .of said condensers in the gridcathode circuit being so chosen that the voltages across them are substantially equal and opposite for at least one value of image frequency.

3. An image rejector and signal selector circuit formed of a single tuned circuit only comprising, in combination, an inductance, a pair of condensers connected in series with each otheracross said inductance to form a closed loop circuit, a third condenser having one terminal connected to the common point of said series connected condensers and the other terminal connected to ground, an antenna connected to one terminal of said inductance and feeding energy to said tuned circuit through one path only, a vacuum tube amplifier having a cathode and a control grid, said cathode being connected to ground and said control grid being connected to the other terminal of said inductance, said circuit being tunable to desired signal frequency and the values of said condensers connected between control grid and ground being so chosen that the voltages across them are substantially equal and opposite for at least one value of image frequency.

4. In an image rejector and signal selector circuit formed of a single tuned circuit only comprising, in combination, an inductance, a pair of condensers connected in series with each other across said inductance to form a closed loop circuit, a third condenser having one terminal connected to the common point of said series connected condensers, an input circuit connected across said third condenser and one of said series connected condensers and feeding energy to said tuned circuit through one path only, a vacuum tube amplifier having a cathode and a control grid, the control grid-cathode circuit of said amplifier being connected across said third condenser and the other of said series connected condensers, said inductance being variable to tune said circuit to desired signal frequency and one of the condensers in said grid-cathode circuit being variable to make the image frequency voltage across it and across the other condenser in said grid-cathode circuit equal and opposite for at least one value of desired signal frequency.

5. An image rejector and signal selector circuit formed of a single tuned circuit only comprising, in combination, an inductance, a pair of condensers connected in series with each other across said inductance to form a closed loop circuit, a third condenser having one terminal connected to the common point of said series connected condensers, an input circuit connected across said third condenser and one of said series connected condensers and feeding energy to said tuned circuit through one path only, a vacuum tube amplifier having a cathode and a control grid, the control grid-cathode circuit of said amplifier being connected across said third condenser and the other of said series con-nected condensers, one of the elements of said closed circuit being variable to tune said circuit, one of the condensers in said grid-cathode circuit being variable to maintain the image frequency voltage across said condensers in said grid-cathode circuit substantially equal and opposite and means for varying said variable elements in unison while maintaining the image frequency voltages across said condensers in said grid-cathode circuit equal and opposite.

6. An image reiector and signal selector circuit formed of a single tuned circuit only comprising, in combination, an inductance, a pair of condensers connected in series with each other across said inductance to form a closed loop circuit, a third condenser having one terminal connected to the common point of said series connected condensers, an input circuit connected across said third condenser and one of said series connected condensers and feeding energy .to said tuned circuit through one path only, a vacuum tube amplifier having a cathode and a control grid, the control grid-cathode circuit of said am plifier being connected across said third con denser and the other of said series connected condensers,one of the elements of said closed circuit being variable to tune said circuit, said third condenser being variable to maintain the image frequency voltage across said condensers in said grid-cathode circuit substantially equal and opposite, and means for varying said variable elements in unison while maintaining the image frequency voltages across said condensers in said grid-cathode circuit equal and opposite.

7. An image rejector and signal selector circuit formed of a single tuned circuit only comprising, in combination, an inductance, a pair of condensers connected in series with each other across said inductance to form a closed loop circuit, a third condenser having one terminal connected to the common point of said series connected condensers, an input circuit connected across said third condenser and one of said series connected condensers and feeding energy to said tuned circuit through one path only, a vacuum tube amplifier having a cathode and a control grid, the control grid-cathode circuit of said amplifier being connected across said third condenser and the other of said series connected condensers, said inductance being variable to tune said closed circuit, said third condenser being variable to maintain the image frequency voltage across said condensers in said grid-cathode circuit substantially equal and opposite, and means for varying both said variable elements in unison while maintaining the image frequency voltages across said condensers in said grid-cathode circuit equal and opposite.

8. An image rejector and signal selector circuit formed of a single tuned circuit only comprising, in combination, an inductance, a pair of condensers connected in series with each other across said inductance to form a closed loop circuit, a third condenser, having one terminal connected to the common point of said series connected condensers, an input circuit connected across said third condenser and one of said series connected condensers and feeding energy to said tuned circuit through one path only, a Vacuum tube amplifier having a cathode and a control grid, the control grid-cathode circuit of said amplifier being connected across said third condenser and the other of said series connected condensers, one of said series connected condensers being variable to tune said closed circuit and said third condenser being variableto maintain the image frequency voltage across said condensers in said grid-cathode circuit substantially equal and opposite, and means for varying said condensers in unison While maintaining the image frequency voltages across said condensers in said grid-cathode circuit equal and opposite.

9. An image rejector andsignal selector circuit formed of a single tuned circuit only comprising, in combination, an inductance, a pair'off condensers connected inseries with each other across said inductance to form a closed loop circuit, a third condenser having one terminal connected to the common point of said series connectedcondensers, an input circuit comprising the plate-cathode circuit of a first vacuum tube amplifier connected across said third condenser and one of said series connected condensers and feeding energy to said tuned circuit through a the other of said series connected condensers,

said circuit being tunable todesired signal frequency and. the values of said condensers in the grid-cathode circuit being so chosen that the voltages across them are substantially equal and opposite for at least one value of image frecondensers connected in series with each other across said inductance to form a closed loop circuit, a third condenser having one terminal connected to the common point of said series connected condensers, an input circuit connected across said third condenser and one of said series connected condensers and feeding energy to'said tuned circuit through a single path only, a vacuum tube amplifier having a cathode and a control grid, the control grid-cathode circuit of said amplifier being connected across said third condenser and the other of said series connected condensers, one of the elements of said closed.

tuned circuit being variable to tune said circuit cuit, and said third condenser being variable to vary the magnitude of image frequency voltage across it without substantially affecting the frequency of said tunedccircuit whereby image frequency voltages across said condensers in said grid-cathode circuit may be made substantially equal and opposite.

FULTON CUTTING.

VERLIS WILEY.

CERTIFICATE OF CORRECTION.

Patent No. 2,2 6,277.- September 16, 19in.

FULTON CUTTING, ET AL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as followsz Page 2 'second column, line 75, claim 1, for the word "turnable" read --tunab1e--; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this LLth day of November, A. D. 191A.

Henry Van Arsciale,

(Seal) 'Acting Commissioner of Patents. 

